The present invention relates generally to an imaging system utilizing imaging sheets having a surface coating of rupturable photosensitive microcapsules and, more particularly, to a method and apparatus for processing such imaging sheets.
U.S. Pat. Nos. 4,440,846 and 4,399,209, which are assigned to The Mead Corporation and hereby incorporated by reference, describe an imaging system wherein a photosensitive layer comprising microcapsules containing a photosensitive composition in the internal phase is image-wise exposed to actinic radiation and subjected to a uniform rupturing force whereupon the microcapsules rupture and image-wise release the internal phase. The imaging system is particularly advantageous because it is a totally dry system and does not rely upon the application of wet development processing solutions to produce the image. An image-forming chromogenic material, such as a substantially colorless color former, is typically associated with the microcapsules. When the microcapsules rupture, the color former image-wise reacts with a developer material and produces a color image. In the embodiments described in the referenced patents, the microcapsules are typically ruptured by passing image-wise exposed imaging sheets through the nip between a pair of calender rollers.
While heavy pressure is not required to rupture the microcapsules, high pressure and large calender rollers are normally used to develop the imaging sheets. Even carefully machined metal calender rollers have uneven surfaces. If one roller is simply rested upon another, the surfaces of the rollers are not in contact over the entire length of the rollers. By applying pressure to the rollers, the uneven surfaces or surface irregularities are "smoothed out" to provide a uniform contact line between the rollers. The high pressure and large size of the rollers are necessary to achieve a uniform distribution of the rupturing force across the surface of the imaging sheets. If the rupturing force is not uniformly distributed, the imaging sheets develop unevenly and the tonal characteristics of the resulting images are not good.
As the width of the imaging sheets and the corresponding length of the calender rollers increases, the diameter of the rollers must also be increased to maintain sufficient stiffness such that pressure applied to the ends of the rollers is distributed evenly across the entire expanse of the rollers. As a general rule, as the length of the rollers is doubled, the diameter of the rollers must be doubled in order to maintain sufficient stiffness. Thus, as the size of the imaging sheets increases, the cost and size of effective pressure type development apparatus becomes prohibitive. In particular, while larger pressure rollers may be accommodated in development apparatuses designed to develop relatively small copies, e.g., 8 to 12 inches in width, in certain applications such as color proofing, large copy widths, sometimes in excess of 36 inches, are used. These copies are so large as to make it impractical to develop them using pressure rollers.
An additional drawback of pressure processing imaging sheets is that when high pressures are applied to the calender rollers, as required to overcome surface irregularities and achieve uniform development across the rollers, some of the midtone quality of the developed images is lost. This is apparently due to a loss of a differential microcapsule rupture and is referred to as "midtone mottle". Thus, a number of difficulties have been encountered in designing pressure type development apparatus for the aforementioned imaging sheets.
An improved arrangement for processing imaging sheets without high pressure and bulky calender rollers is disclosed in commonly assigned U.S. Pat. No. 4,448,516, which is hereby incorporated by reference. In this patent, the imaging sheets are passed over a developer roll having a fibrous outer surface composed of interwoven natural or synthetic fibers or flexible bristles. The developer roll is rotated in contact with the imaging sheets to rupture the microcapsules on the sheets. The developer roll is an improvement over the bulky high pressure calender rollers and has advantages over the alternative proposals disclosed in the first referenced patents.
Unfortunately, the fibrous outer surface of the developer roll of the last referenced patent has limited microcapsule engaging surface area. Accordingly, as imaging sheets are processed by the developer roll to rupture the microcapsules on the sheets, oil released from the ruptured microcapsules accumulates on the limited surface area of the developer roll. After a given amount of oil accumulation, background coloring (fog) begins to increase on processed imaging sheets. Since background coloring detracts from the developed image, the developer rolls must be frequently changed to maintain high quality images.
New alternatives which offer inexpensive techniques for developing imaging sheets, overcome deficiencies of the prior art and may be preferred for selected applications are in demand and serve to advance the art of imaging systems utilizing the imaging sheets of the first referenced patents.